As-processed metals and alloys by selective laser melting (SLM), or laser powder bed fusion (L-PBF), are often full of defects and flaws such as dislocations, twins, elemental segregations, impurities and porosities, which can positively or negatively impact mechanical properties. Here, we systematically characterize the tensile behavior of L-PBF Ti-6Al-4V at quasi-static strain rate and room temperature, including state-of-the-art in situ synchrotron X-ray diffraction (SXRD) and computed tomography (SXCT). These studies reveal that the tensile yield strength and uniform elongation are mainly dictated by the as-built microstructure, while the strain-to-failure is sensitive to the porosity, even in very high-density samples (> 99.5%). in situ SXRD reveals that the micro-plasticity in as-built Ti64 initiates at a stress level well below its macroscopic yield strength, signified by the early lattice strain deviation behavior of {0002} and {11 2¯ 0} reflections. SXCT reveals pore growth mechanisms when the tensile axis is perpendicular to the build direction, whereas no such behavior is observed as the tensile axis is along the build direction. These anisotropic pore growth mechanisms result in vast differences in the strain-to-failure of L-PBF materials. Our melt-pool dynamics modeling with similar laser conditions to the experiments identifies a previously unknown pore source; ie, edge-of-track pores. We present a normalized energy diagram to identify the optimized processing window for high quality samples.